Stiles' account of two-color increment threshold data has served for many years as a convenient scheme for organizing detection and adaptation results. However, various tests of this account - including some by Stiles himself - require that the original, simple proposal be modified. Some authors have suggested that a zonal theory, based on opponent-colors analysis might serve as a useful replacement. A major difficulty in studying these zonal theories is that there are no agreed upon diagnostic procedures for identifying which zonal-theoretic pathway is mediating detection of a test signal. Based upon an analysis of the predictions of zonal theories, I propose to test the hypothesis that wavelength discriminability at threshold may serve as such a diagnostic procedure. Assisting the successful completion of that project, I then propose to study a specific bypothesis concerning the adapting behavior of the non-opponent pathway postulated by zonal theories.